Rotork Controls
has established a global reputation for robust and reliable valve
actuators, operating in some of the worlds most demanding
and critical applications. Thorough testing of the motors used
in the actuators is one of the keys to this assured performance,
with TorqSense transducers from Sensor Technology playing a vital
role in the test process.

Rotork is the
worlds leading manufacturer of electric, pneumatic and
hydraulic valve actuators and associated control systems, valve
gearboxes and valve accessories. The companys electric
valve actuator division offers an electrical solution to industrial
valve control and actuation applications of virtually any size,
description and complexity.

Its capabilities
encompass actuators and control systems for new plants, plant
extensions and upgrades, together with life-of-plant support.
The divisions unrivalled range of products offers a robust
and economic solution for every valve duty and operating environment.

Rotork Controls
chief development engineer Geoff Beeho comments: If you
look at something like an oil line, then any failure of the valve
actuator could potentially be costing millions of dollars a day.
We have to be able to guarantee the performance of the actuators
in the most demanding conditions, so testing is vital. TorqSense
torque transducers from Sensor Technology are a key element of
our test rigs.

Companies and
businesses around the world rely on Rotork actuators, in applications
as diverse as power generation, water and sewage, industrial
processes, subsea, marine, HVAC, incinerators and cement plants.
Common features in many of these applications are challenging
environmental conditions, extremes of temperatures, and high
costs associated with any downtime. Because of this, absolute
reliability of the valve actuators is vital.

Rotork Controls
designs custom actuator designs built around motorised gearboxes,
using non-standard open frame motors. We use a variety
of motors depending on the particular application, including
DC motors, single-phase motors and three-phase motors, but we
dont rely on off-the-shelf products. Instead, we use open
frame motors that we integrate directly into our own systems.

Performance
testing is very important, he continues. The duty
cycle is atypical  our motors are not in continuous use,
only running intermittently and for short period. What is important
to us is the short term ratings, and this data isnt always
readily available. We have to have performance data that will
tell us exactly what we need to know about the motors in these
most demanding applications.

Rotork Controls
designed its own rig to test the motors, with Sensor Technology
TorqSense transducers at its heart. We needed non-contact
torque sensors in the rig. The early design used magnetically
coupled torque sensors but we were never completely happy with
these. So when we upgraded the rig to be able to perform a wider
range of tests, we took the opportunity to look around the market
and we found the TorqSense transducers.

TorqSense transducers
are non-contact devices that operate using surface acoustic wave
(SAW) principles. In a TorqSense transducer, surface waves are
produced by passing an alternating voltage across the terminals
of two interleaved comb-shaped arrays, laid onto one end of a
piezoelectric substrate. A receiving array at the other end of
the transducer converts the wave into an electric signal.

The frequency
is dependent upon the spacing of the teeth in the array and as
the direction of wave propagation is at right angles to the teeth,
any change in its length alters the spacing of the teeth and
hence the operating frequency. Tension in the transducer reduces
the operating frequency while compression increases it. To measure
the torque in a rotating shaft, two saw sensors are bonded to
a shaft at 45deg to the axis of rotation. When the shaft is subjected
to torque, a signal is produced which is transmitted to a stationary
pick up via a capacitive couple comprising two discs, one of
which rotates with the shaft, the other being static.

The TorqSense
transducers immediately struck us as being a much more robust
solution, better suited to handling the torques we were subjecting
them to, comments Beeho.

The common practices
of over-specifying a motor that guarantee reliable operation
in standard applications simply dont apply in valve actuation.
Motor sizing is critical, says Beeho. If we
put in a motor that is too big, then we can risk damaging the
valve. We can use motors that appear to be under-rated, but because
our duty-cycle is so short we can get better performance from
the motor. But the performance data to support this sort of motor
use isnt always available, or isnt always reliable,
so we need to perform our own tests. We also need to know how
our motors will handle extremes of temperature in given applications
so that reliability is never compromised. The TorqSense transducers
allow us to make all the required tests.

An advantage
of the TorqSense transducers in the Rotork test rig is that they
provide both analogue and digital outputs. This has proved
to be a real bonus, says Beeho. Not only can we use
one output to check the calibration of the other, but it also
means we have the flexibility to use both outputs performing
different tests. For example, we might use the analogue output
to feed torque and speed data into the power analyser during
a performance test, and then use the digital output for a temperature
test. Here, we run the motor for 15 minutes at constant torque
and see how hot it gets. The output from the TorqSense transducer
allows us to close the loop so that we can keep the torque constant.

In any application,
the fundamental considerations are the actuators ability
to overcome the resistive forces of the valve, and the dynamic
performance requirements to enable the valve to adequately meet
the process demands. With its comprehensive testing procedures
built around the TorqSense transducers, Rotork Controls has the
confidence that its electrically operated valve actuators will
always meet the application requirements.